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Date: Wed, 17 May 2017 21:15:05 +0000
From: mzpqnxow <musl@...qnxow.com>
To: musl@...ts.openwall.com
Subject: Re: Question about setting argv[0] when manually using dynamic linker
Does the shell built-in "exec" -a app1 /bah/dynamic-loader.so
/path/to/app1" work or it has the same problem with argv[0] getting reset?
On Wed, May 17, 2017 at 15:16 John Regan <saxindustries@...il.com> wrote:
>
> On Wed, May 17, 2017 at 2:07 PM, <u-uy74@...ey.se> wrote:
>
>> On Wed, May 17, 2017 at 12:24:28PM -0400, Rich Felker wrote:
>> > > >>> On Tue, May 16, 2017 at 08:38:56PM -0400, John Regan wrote:
>> > > >>> > Hi there - I was wondering if it's possible to somehow set
>> argv[0] when
>> > > >>> > calling the dynamic linker to load a program.
>>
>> > > >>> Set argv[0] to whatever you need when you exec*() the dynamic
>> loader,
>> > > >>> which is straightforward with a binary wrapper (not with a shell).
>> > > >>>
>> > > >>> A binary wrapper also adds less overhead then going through a
>> shell.
>>
>> > > >>> Rune
>>
>> > a completely reasonable and recommended way for deploying dynamic
>> > linked apps in a self-contained way that doesn't depend on musl libc
>> > on the host. Unfortunately there's no way to set argv[0] like you want
>>
>> We do deploy dynamic linked apps without any dependencies on the libraries
>> on the host. It works just fine with musl-as-it-is, including the
>> questionably designed applications like busybox and gcc who
>> analyze argv[0].
>>
>> > at this time. Perhaps adding an option like --argv0=foo would be
>> > appropriate.
>>
>> What would be the justification for adding the supporting code (to every
>> instance of the dynamic loader)?
>>
>> It looks like --argv0=foo is meant to overcome a specific limitation in
>> bourne shell, in a specific context where the task can be solved easily
>> and generally better without involving the bourne shell in the first hand.
>>
>> I would like to see an example of a situation where a wrapper in C (or
>> any language allowing setting of argv[0]) is less appropriate?
>>
>> If one really has a reason to express the wrapper in sh, a one-liner in
>> C and an extra exec from the shell (much cheaper than starting the
>> shell itself was) is sufficient to make it work.
>>
>> Rune
>>
>> Hi Rune - would you mind sharing some tips on doing that?
>
> I wrote and compiled a short program that just dumps the elements in argv,
> then a wrapper program that figures out the needed paths for libc, real
> binary, etc, but it seems like argv[0] gets reset by the dynamic loader.
>
> I'm calling execve with the path to the libc.so, and argv is somenthing
> like:
>
> argv[0] - desired process name
> argv[1] - full path to the real binary
> argv[2...] arguments
>
> The 'real' binary is loaded and ran, but winds up printing out:
>
> argv[0] - full path to the real binary
> argv[1...] arguments
>
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